The invention relates to a method for measuring mechanical workpieces by tomography.
In the production of, in particular, high-quality mechanical workpieces, it is necessary for the workpiece to be measured after or even during the production and processing in order to check if certain measurement points on the workpiece defined in advance meet predefined dimensions within some tolerances likewise defined in advance. For this purpose, use is often made of multi-coordinate measurement machines, which scan the workpiece by means of touch probes and monitor the dimensional accuracy of the workpiece surfaces in this fashion.
As an alternative, a different measurement approach has been used for some time now for measuring mechanical workpieces, which approach was initially used in medicine as an imaging method for examining human bodies, namely the method of computer tomography (CT). In medical applications, the body or body region to be examined is irradiated in a plane by means of a linear array of X-ray radiation sources. On the opposite side of the body, a corresponding array of X-ray detectors is situated opposite the array of X-ray radiation sources. This pair of arrays is then rotated by an angular step about an axis running perpendicular to the plane, and a further recording is produced. After the array has been rotated, step-by-step, through a total of 360°, a cross-sectional image in the plane is computed from the individual recordings, which image reproduces the density distribution in this plane. If the body and the pair of arrays are now subsequently displaced relative to one another by a linear step along the axis, a further, immediately adjacent cross-sectional image can be generated and a three-dimensional display of the body or body region can be generated from a plurality of such adjacent cross-sectional images. This measurement method is rather complicated because a human body has a very complex density distribution with density varying in large regions, and the structures to be recorded can differ significantly and can be of unforeseeable type and shape.
By contrast, workpieces in quality control are often objects with only two densities, namely the density of the material of the workpiece and the density of air. Furthermore, the structures to be examined during quality control are known and merely have to be examined with respect to deviations.
DE 10 2005 039 422 A1 describes a computer tomography method, simplified with respect to medical applications, for examining workpieces. In this method, a mechanical workpiece is situated on a rotary table between a spot-like X-ray radiation source and an areal detector array. Here, the rotational axis of the rotary table runs substantially perpendicular to the radiation direction. The workpiece is penetrated by the X-ray radiation, and a shadow image of the workpiece is created on the detector array. The workpiece is then successively rotated by an angular step, for example 800 or 1200 times, on the rotary table and additional shadow images are created. A three-dimensional image of the workpiece is then computed from the plurality of shadow images, for example according to a method of back-projection, as described in DE 39 24 066 A1.
Computer tomography measuring stations of this type are commercially offered, for example by the assignee of the invention under the brand name “Metrotom” (www.zeiss.de/imt).
With prior art measuring stations, a so-called “reconstruction” is performed, i.e. a complete three-dimensional image of the density distribution of the measurement object (mechanical workpiece) is computed with a predefined resolution of volume elements (voxels). In order to be able to do this with sufficiently high precision for workpiece measuring, a large number of individual images are required, in practice between 360 and 1080, corresponding to angular steps of the rotary table between 1° and ⅓°. However, a certain amount of time is required for each rotational step in order to accelerate the rotary table from its rest position, to move it, and to decelerate it again until it rests. Furthermore, a certain amount of time is required to calculate the three-dimensional image, even though this can already be started during the measurement acquisition, and so, overall, prior art measuring stations require between 15 and 30 minutes for a complete measurement. In many cases, this measuring time is unacceptable.